asp.net - How does IsPostback technically work?

Question

I'm currently having a strange issue whereby all browsers except from Google Chrome are registering a call to IsPostback within a Page_Load event as true when I click an asp.net button which simply posts back to the same page.

This has led me to try and discover how the IsPostback property within an ASP .Net page is technically implemented, something I'm struggling to find.

My thoughts to date are that it could be related to the following;

• The request VERB type is POST rather than GET.
• The hidden input containing the Viewstate information has no information present and therefore no previously submitted control information is available.
• The http referer in the request headers is the same as the current URL.

Can anyone provide an actual breakdown of the conditions used to determine the IsPostback boolean property?

Note: I'm looking for the actual implementation rather than perceptions / theory as I'm hoping to use this to actively resolve an issue. I've also searched MSDN and to date cannot find any technical article accurately covering the mechanism.

Thanks in advance, Brian.

Answer 1

The page looks for the existence of a __PREVIOUSPAGE form value.

From Reflector:

public bool IsPostBack
{
    get
    {   //_requestValueCollection = Form or Querystring name/value pairs
        if (this._requestValueCollection == null)
        {
            return false;
        }

        //_isCrossPagePostBack = _requestValueCollection["__PREVIOUSPAGE"] != null
        if (this._isCrossPagePostBack)
        {
            return true;
        }

        //_pageFlags[8] = this._requestValueCollection["__PREVIOUSPAGE"] == null
        if (this._pageFlags[8])
        {
            return false;
        }

        return (   ((this.Context.ServerExecuteDepth <= 0) 
                || (   (this.Context.Handler != null) 
                    && !(base.GetType() != this.Context.Handler.GetType())))
                && !this._fPageLayoutChanged);
    }
}